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Simple Probing Tool for Corporate Walled Garden Networks

The Problem: Network sensors such as Suricata or Bro/Zeek have been successfully deployed in a large network, but the rate of alarms or other useful information is suspiciously low -- not even the usual background noise can be seen. Can we be sure that our sensors are fed all the relevant traffic?

An attempt at a solution: Let's generate some network traffic and see if we can transmit some magic strings to and from the outside world beyond our walled garden network -- and if we are able to detect those using our sensors.

Operation

From a user perspective:

1. Generate a server configuration file that defines TCP ports and magic strings to exchange (see below for an example). Configure matching alerting rules in the network sensors.
2. Start the server on a publicly accessible host.
3. Start clients with the -server parameter pointing to one of the HTTP ports served by the server.
4. Analyze logs generated by the server and the network sensors.

The server writes its log output to standard error.

What happens behind the scenes:

1. On startup, both server and client generate private/public NaCL key pairs.
2. The client posts its public key to the server and receives the server's public key
3. The client requests the server's configuration. The configuration is signed/encrypted to circumvent tampering by middleboxes.
4. The client runs a few experiments, expecting every configured magic strings to be correctly transmitted via a special header, a Cookie or Set-Cookie header, the message body.
5. The client posts its findings to the server.

Example server configuration file

{
    "hostname": "mauerspecht.example.com",
    "http-ports": [8080, 18080],
    "magic-strings": [
        "unique-match-string-18475910",
        "START_KEYLOGGER",
        "X5O!P%@AP[4\\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*"
    ]
}

Command line parameters

Client:

  -server string
    	Server URL (default "http://localhost:8080")
  -proxy string
    	Proxy URL

Server:

  -config string
    	Config file (default "mauerspecht.json")

Building

If a Go toolchain with modules support is available (version 1.11 or later), simply running make all from the Git checkout is sufficient.

For earlier Go versions, run go get github.com/DCSO/mauerspecht change to the directory within the GOPATH where the project was downloaded, and run make all

In both cases, the following binaries will be generated:

• mauerspecht-server: The server component, a Linux/x86-64 binary
• mauerspecht-client-$ARCH: The clients, for various architectures

Limitations, possible future features

• HTTPS -- self-signed server certificates, possible use of client certificates
• Non-HTTP protocols (IRC?)
• The server stores session keys submitted by clients in memory and does not expire them yet. This is a denial-of-service vector.
• Bundled client configuration for easy single-binary deployment (see also: spyre)

Author

Hilko Bengen <hilko.bengen@dcso.de>

Copyright

Copyright 2019 Deutsche Cyber-Sicherheitsorganisation GmbH

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.